ndenter size effect on the incipient plasticity of Al (001) surface is studied by using the quasicontinuum simulation method. Two cylindrical indenters with the radii 2.5nm and 40nm are used to penetrate the surface respectively, and in displacement-control in steps of 0.02 nm. Results show that the plasticity under the small indenter is activated by discrete dislocation nucleation events, while the plasticity under the large indenter is dominated by a collective dislocation activity. Contact pressure calculations reveal that reversible incipient plasticity occurs under the small indenter, i.e. the plastically deformed surface can completely recover upon withdrawal of the indenter, while the incipient plasticity under the large indenter seems to be irreversible.

Dislocation core structures in Au and Cu crystals are investigated by means of quasicontinuum simulations combined with the embedded atom method potentials. A dislocation pair in a graphene sheet, which is observed by Warner et al. experimentally, is also analyzed in the present work. The strain fields around these dislocations in Au, Cu, and graphene crystals are calculated by analyzing the coordinates of discrete atoms, which is a strain tensor calculation method proposed by Zimmerman et al., and compared with theoretical predictions based on Foreman dislocation model. It is shown that the strain fields given by Zimmerman theory are completely suitable for describing the dislocation core structures of Au, Cu and graphene crystals. However, compared with the results of Au and Cu, the Zimmerman strain field in the vicinity of graphene dislocation core is a little less accurate, possibly due to the effect of lattice symmetry of graphene, which needs to be clarified in the future study.

Indenter size effect on the reversible incipient plasticity of Al (001) surface is studied by quasicontinuum simulations. Two cylindrical indenters with the radii 2.5nm and 17.5nm are used to penetrate the surface respectively, in displacement-control in steps of 0.02 nm. Results show that the plasticity under the small indenter is reversible, since it is dominated by the nucleation of a thin deformation twin, which can be fully removed after withdrawal of the indenter, due to the imaging force and stacking fault energy. Under the large indenter, multiple slip systems are activated simultaneously when incipient plasticity occurs, a few twin, dislocation and stacking fault ribbons still remain under the surface when the indenter has been completely retracted, thus the plasticity is irreversible.